Every household needs one, but there are 3 in mine - a big expensive one, a robot and a handheld mini.
Here is how each of them contribute to keeping my house clean.
1. Rainbow vacuum: it cost more than 3 grand SGD, but the reason I bought it is because it is one of the best cleaning solutions I've experienced. It is bulky and takes some effort to lug around the house, not to mention the cable which has no auto function to be kept and has to be manually rounded up. But the camel hair brushes and water based functionality is very effective in cleaning the floor, beds, upholstery, and even the ornaments in our house. Also relatively easy to upkeep and removes dust so well I only have to mop once a month!
I recommend to buy this vacuum only if you have people at home who have sensitive noses that react to dust-mites. If you have a helper, frankly any vacuum is fine unless you need one that can help you clean curtains, beds etc on a regular basis, but there are other solutions like the Raycorp that is less bulky and work equally well in killing mattress dust-mites. If you are considering getting a dolphin vs a rainbow, trust me, the rainbow is better. I have done much research before buying my rainbow and I just do not like the Dolphin's sales methods and I find their after sales service is bad.
2. Samsung Navi-bot: my husband thinks this is the best robotic vacuum cleaner on the market. We used to use it everyday before baby came to keep our floors clean. It is cheaper than I-Robot but smarter and is good at covering most spots in our little home, especially under the sofa and bed etc. Unfortunately our Navi is underused nowadays coz of the big world map mat in the middle of our living room makes it hard for him to move around like he used to. But I think I'll start reviving him just for my baby's amusement
3. Rowenta Extenso Wet & Dry vacuum (handheld): we just bought this as I wanted a handheld for easy cleaning of hair and other small messes (breadcrumbs etc). Chose this Rowenta as it can clean both wet and dry messes, charges fast and is relatively fuss free to empty and upkeep.
If you need any further advice on vacuum choices, feel free to ask before you make your cleaning investment.